1. Field of the Invention
The present invention relates to an electromagnetic induction type absolute position measuring encoder. In particular, the present invention relates to an electromagnetic induction type absolute position measuring encoder suitable for use in a caliper, an indicator, a linear scale, a micrometer, or the like, and capable of achieving a high precision in measurement due to an improvement in S/N ratio and/or capable of reducing a size thereof due to a reduction in scale width and therefore in encoder width.
2. Description of the Related Art
As described in Japanese Patent Application Laid-Open No. Hei. 10-318781 (hereinafter referred to as Patent Literature 1) or in Japanese Patent Application Laid-Open No. 2003-121206 (hereinafter referred to as Patent Literature 2), there has been known an electromagnetic induction type encoder as that of FIG. 1 showing an example of Patent Literature 2. The electromagnetic induction type encoder includes: a number of scale coils 14 and 16 arranged on a scale 10 along a measuring direction; and transmitter coils 24 and 26 and receiver coils 20 and 22 provided on a grid (also referred to as a slider) 12 capable of moving relative to the scale 10 in the measuring direction. The electromagnetic induction type encoder detects a relative displacement between the scale 10 and the grid 12 on the basis of a flux change detected at the receiver coil via the scale coil when the transmitter coil is excited. In the drawing, reference numeral 28 denotes a transmission control section, and reference numeral 30 denotes a receiving control section.
As shown in FIG. 2, in order to reduce an offset, which is an excess signal, in such an electromagnetic induction type encoder, an offset has been reduced by placing the receiver coil 20 at a position where magnetic fields generated by the transmitter coils 24 are canceled out to be net zero (a central portion between the transmitter coils on both sides thereof in the example of FIG. 2). Note that in Patent Literature 2, the second receiver coils 22 are also provided on both sides of the second transmitter coil 26 as shown in FIG. 3 in addition to a configuration formed by the first transmitter coils 24 and the first receiver coil 20 in FIG. 2.
However, this configuration requires three rows of scale coils, and the line of the scale coils is therefore long. Thus, there is a problem that the generated induced current is attenuated due to an impedance of the scale coil itself and it is therefore difficult to obtain a strong signal.
In order to solve such a problem, the present applicant has suggested in Japanese Patent Application Laid-Open No. 2009-186200 (hereinafter referred to as Patent Literature 3) that, as shown in FIG. 4 of the present application corresponding to FIG. 6 in Patent Literature 3, plural sets of transmitter coils 24A and 24B, receiver coils 20A and 20B, and scale coils 14A and 14B are arranged symmetrically with respect to the center of the scale 10, and one (14A, for example) of the scale coils symmetrically positioned with respect to the scale center is placed at a position displaced by a ½ phase of a scale pitch λ from the other scale coil (14B, for example).
Furthermore, as shown in FIG. 5, it is conceivable to place two sets of tracks including scale coils, transmitter coils, and receiver coils provided in a scale width direction (grid width direction) with different scale pitches of λ1 and λ2 so as to measure absolute positions. The two sets are a set of scale pitch λ1 formed by a transmitter coil 24-1 on the lower side of the figure, scale coils 14-1a and a receiver coil 20-1 on the upper side of the figure and a set of scale pitch λ2 formed by a transmitter coil 24-2 on the upper side of the figure, scale coils 14-2a and a receiver coil 20-2 on the lower side of the figure. In this drawing, reference numeral 14-3 denotes a coil for connecting between the scale coils 14-1a and 14-2a (referred to as a connecting coil).
FIG. 6 shows an operation of detecting the scale coil 14-1a on the upper side of FIG. 5 having the scale pitch of λ1 by the receiver coil 20-1. As shown in the drawing, a magnetic field generated by the driving of the transmitter coil 24-1 with a driving current ID leads to the generation of an induced current Ia at the scale coil 14-2a. Then, a magnetic field generated by the current Ia flowing through the scale coil 14-1a via the connecting coil 14-3 is detected at the receiver coil 20-1.
On the other hand, FIG. 7 shows an operation of detecting the scale coil 14-2a on the lower side of FIG. 5 having the scale pitch of λ2 by the receiver coil 20-2. As shown in the drawing, a magnetic field generated by the driving of the transmitter coil 24-2 with the driving current ID leads to the generation of the induced current Ia at the scale coil 14-1a. Then, a magnetic field generated by the current Ia flowing through the scale coil 14-2a via the connecting coil 14-3 is detected at the receiver coil 20-2.
With the configuration of FIG. 5, however, the receiver coil 20-1 (20-2) and the transmitter coil 24-1 (24-2) need to be placed at positions spaced apart from each other in order to reduce a direct crosstalk amount from the transmitter coils 24-1 and 24-2 to the receiver coils 20-2 and 20-1 on the grid 12. Thus, a length of the scale coils on the scale 10 (a length of the scale coil 14-1a+a length of the scale coil 14-2a+a length of the connecting coil 14-3) becomes long, resulting in an attenuation of the generated induced current Ia due to the impedance of the scale coil itself. Thus, there is a problem that a strong signal is difficult to be obtained.